A headlamp of a vehicle is used to illuminate a front side of the vehicle during operation of the vehicle, where the headlamp is disposed at an inside thereof with a light source, and an upper side or a lower side of the front of the vehicle is illuminated with a light emitted from the light source. The headlamp is laid in a high temperature environment caused by the light source itself of the headlamp and heat generated from an engine of the vehicle, and there is generated a difference between a temperature inside the headlamp and an outside of the vehicle, which causes generation of dewing inside the headlamp.
That is, as illustrated in FIG. 1(a), when the headlamp is initially lighted, a dew point temperature inside the headlamp is evaporated by a high temperature environment in response to the lighting of the headlamp to allow the headlamp to be in a temperature-increased state over an outside.
However, when a certain time lapses after light-off of the headlamp, higher and lower area temperatures inside the head lamp descend together to allow the dew point temperature to go down at the same time. At this time, a temperature reversal phenomenon is generated where an air descending speed at the lower area inside the headlamp is further increased over a descending speed of dew point temperature due to sealing treatment of the headlamp, to thereby generate moisture inside the headlamp.
The moisture generating problem inside the headlamp can create a defect on the light source of the headlamp and degrade the merchantability of a vehicle, which is recognized as a chronic problem in a vehicular headlamp system. In order to solve the aforesaid problem, various solutions have been proposed but the situation is that no fundamental solution has been made.
For example, although there is a dew prevention method in which an inner surface of a headlamp lens is coated with a dew prevention coating, or there is a dew prevention method using a silica gel, these methods are not fundamental solutions and create problems in life time, performances and reliability. Thus, necessity is required to fundamentally prevent the generation of dew.